The present invention is directed to a process of forming a nitro compounds by gaseous phase reaction of a carboxylic acid with nitrogen dioxide. The present process provides a method to form pre-selected nitro-compounds based on the particular carboxylic acid feed. The process further alleviates certain processing steps required in prior art nitration of hydrocarbons.
Processes to form nitroparaffins by gaseous phase nitration are known. U.S. Pat. Nos. 3,780,115 and 3,869,253 teach that nitration of saturated hydrocarbons higher than methane can be accomplished by contacting the hydrocarbon feed with nitrogen dioxide in the presence of oxygen, such as in the form of air. The reactant gases are preheated and then introduced into the reaction zone where the gaseous phase nitration is carried out at elevated pressure and at elevated temperature. The gaseous effluent emitted from the nitration reaction zone is rapidly quenched. The quenched mixture then enters a separator where the gaseous materials are removed for subsequent purification and recycling and the remaining organic and aqueous phase liquid materials are separated by decantation and the nitroparaffins are recovered by distillation. This nitration process yields a mixture of products having a predominance of nitropropanes or nitroethanes.
French Publication No. 78/32,118 discloses that the nitroparaffins product mixture can be made to have an increased yield of nitromethane, the most commercially desired product, by utilizing ethane as the hydrocarbon feed in the homogeneous gas phase nitration. The nitration process can be further enhanced by recycling into the hydrocarbon feed some of the nitropropane product and/or by conducting the nitration in the presence of an inert gas such as nitrogen, hydrogen or argon.
U.S. Pat. No. 4,260,838, similar to the above French reference, teaches that the gas phase nitration process of U.S. Pat. Nos. 3,780,115 and 3,869,253 can be improved by altering the feed stock to obtain suitable percentages of different nitroparaffins as suits the needs of the marketplace. This patent teaches that the feed stock be made up of a mixture containing propane, preferably recycled nitroparaffin and possibly inert gas and/or another alkane. The nitrating agent can be either nitrogen dioxide or nitric acid.
Each of the conventional processes, such as those in the above referenced patents, relies on the use of a hydrocarbon feed which provides a nitroparaffin product mixture. These processes have the further defect of providing low yield of nitroparaffin mixture and low selectivity of the most commercially desired compound, nitromethane. Finally, because of the low yield, processes which are based on the gaseous phase nitration of saturated hydrocarbons produce a large volume of gaseous reaction effluents composed predominantly of unreacted hydrocarbon feed mixed with nitric oxide, carbon monoxide, carbon dioxide and inert diluent gas. The unreacted hydrocarbons must be separated and recovered from the remaining gases, such as by cryogenic means, and then recycled as part of the process feed. Such separation and recovery requires additional equipment and adds to the processing costs of the prior known processes.
A method for selectively forming particular nitroalkanes and nitroaromatics is described in U.S. Pat. No. 3,689,576. The process requires the initial formation of an acyl nitrate by the slow addition of nitric oxides or nitric acid to an acid anhydride alone or in the presence of a small amount of free acid under low temperature conditions and then thermally treating the acyl nitrate to convert it to a nitroalkane or nitroaromatic. This method has not found favor as it requires the formation and use of highly explosive acyl nitrates.
A method to selectively form particular nitroalkanes or nitroaromatics from easily available and processable feed is highly desired. It is particularly desired to have a process to selectively form nitromethane, a very industrially useful product.